The UK, Germany and France reaffirmed the Russian state was responsible for the nerve agent poisoning of former Russian spy Sergei Skripal and his daughter, the British government said Thursday.

The statement came following Prime Minister Theresa May's meetings with French President Emmanuel Macron and German Chancellor Angela Merkel on the sidelines of the European Council summit in Brussels.

"The Prime Minister provided the President and Chancellor with a detailed update on the investigation into the reckless use of a military nerve agent, of a type produced by Russia, on the streets of Salisbury," it said.

It quoted May as telling the French and German leaders that "there had been a positive identification of the chemical used as part of the Novichok group of nerve agents by our world leading scientists at Porton Down".

May also "outlined our knowledge that Russia has previously produced this agent; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views some defectors as legitimate targets for assassinations," according to the statement.

"The UK, Germany and France reaffirmed that there is no plausible explanation other than that the Russian state was responsible," it added.

Twenty-three Russian diplomats left Britain Wednesday in the single biggest expulsion of foreign diplomats from the UK since the 1970s after Russia refused to provide any explanation for their stockpile of Novichok which Britain says Moscow used in the attempted March 4 murders.

The Skripals remain in stable but critical condition in hospital after being found unconscious on a public bench.

Police officer Nick Bailey, who was also critically injured as one of the earliest security personnel to deal with Skripals, was discharged Thursday after a full recovery.

May earlier said the incident took place "against the backdrop of a well-established pattern of Russian state aggression across Europe and beyond".

Russia has denied any involvement in the incident but also expelled 23 British diplomats in a reciprocal move, accusing the UK of not sharing a sample of the substance for their examination.

May, Macron and Merkel "agreed on the importance of sending a strong European message in response to Russia's actions and agreed to remain in close contact in the coming days."

The leaders also spoke about Iran and reaffirmed their commitment to the 2015 nuclear deal between Tehran and world powers and agreed to hold further discussions in April.